Electrical discharge machining circuitry has advanced from the early stages in which relaxation oscillators were used to provide machining power pulses. Independently timed and controlled pulse generators are now almost universally used and in those generators electronic switches are employed in the form of solid state switches or banks of parallel connected switches, particularly transistors. These switches are used to provide the machining power pulses to the gap.
In the electrical discharge machining process, sometimes hereinafter referred to as "EDM", it is necessary that as the workpiece material is removed, a predetermined gap be maintained between the tool electrode and the workpiece through a servo feed system which provides a continuous advance into and toward the workpiece as the material removal progresses. During the electrical discharge machining process, a fluid coolant is circulated through the machining gap to flush the removed workpiece particles from the gap. The coolant is usually furnished under pressure by a pump through one or more openings provided in the electrode and/or workpiece.
One defining characteristic of electrical discharge machining is that the coolant is a dielectric fluid such as kerosene, transformer oil, distilled water, or the like. The dielectric fluid is broken down in minute, localized areas by the action of the machining power pulses, passed between the closely opposed surfaces of the tool electrode and the workpiece. For control of the servo feed system, there is generally utilized an electrical signal from the machining gap in order to control the rate and the direction of servo feed. In many cases, this gap signal is compared to an adjustable reference voltage so that the machine operator can select the rate of servo feed desired.
In precision EDM, it is necessary to precisely control current output from the power module connected to the gap. This control has been generally maintained in the prior art by switching one or more power limiting resistors into or out of circuit with the machining gap. This necessarily results in a non-linear type of current control. Additional problems arise from the relays involved in switching the resistors in series with the gap. The present invention, for the first time, makes possible a linear gap current control in a novel and simplified manner.
It will be understood in the specification that when we refer to "electronic switch", we mean any electronic control device having several electrodes comprising at least two principal or power conducting electrodes acting to control current flow in the power circuit, the conductivity between the principal electrodes generally being controlled by a control electrode in the switch whereby the conductivity of the power circuit is controlled statically or electrically without movement of mechanical elements within the switch. Included within the definition are transistors in which turn-on is accomplished by a control voltage applied to the transistor control electrode and in which turn-off is accomplished automatically in response to the removal of that control voltage. Also included in the definition are devices of the gate type in which turn-on is accomplished by a control voltage applied to the control electrode, which control voltage may be then removed and in which turn-off is accomplished by application of a subsequent control voltage to the control electrode. An additional class of electronic switches, called "electronic trigger devices", falls within this definition and includes thyratrons, semi-conductor controlled rectifiers, and the like. By electronic trigger device, we mean any electronic switch of the type which is triggered on at its control electrode by a pulse and is turned off by a reverse voltage applied for a sufficient time across its principal electrode.
The present invention further incorporates a particular type of electronic switch known in the art as a power field effect transistor. One such type of transistor, specifically a VMOS power field effect transistor sometimes hereinafter referred to as a power FET is included in the circuits used for out invention. Power FET's appropriate for inclusion in EDM power modules are currently manufactured and sold by Siliconix Incorporated, 401 Broad Hollow Rd., Mellville, N.Y. 11746.